Newton's rings are a thin film optical interference phenomenon manifesting as a series of concentric, multicoloured or alternating light and dark circular bands that appear on a photographic image or scan when a very thin, uniform layer of air becomes trapped between two closely adjacent transparent or semi-transparent surfaces - such as a glass scanner bed and the film or print being scanned, or between the glass surfaces of a glass negative carrier in a darkroom enlarger and the film held within it. The phenomenon takes its name from Sir Isaac Newton, who first described and analysed it in the seventeenth century as part of his broader investigation into the behaviour of light.
The physical mechanism responsible for Newton's rings is thin film interference, a consequence of the wave nature of light. When light strikes the upper surface of the thin air layer trapped between the two surfaces, a portion of that light is reflected back upward from the upper boundary of the air layer, while another portion passes through the air gap and is reflected back upward from the lower boundary. These two reflected beams have travelled slightly different path lengths - differing by twice the local thickness of the air gap - and when they recombine, they interfere with one another either constructively or destructively depending on the relationship between their path length difference and the wavelength of the light involved. Where the path length difference corresponds to a whole number of wavelengths, constructive interference produces a bright ring, while where it corresponds to a half number of wavelengths, destructive interference produces a dark ring. Because the wavelength of light varies with colour, the interference conditions are met at different radii for different colours, producing the characteristic multicoloured concentric ring pattern in white light illumination.
In scanning, Newton's rings are a common and frustrating artefact that degrades the quality of scanned images by overlaying the circular interference pattern on top of the image content, where it appears as a distracting and visually obvious banding that cannot be removed in post processing without damaging the underlying image detail. They are most likely to occur when scanning film mounted in or on a flat glass carrier, as the combination of the flat glass surface and the relatively flat film creates ideal conditions for trapping a thin, uniform air layer of the critical thickness required to produce the interference pattern.
Several practical strategies are used to prevent or minimise Newton's rings in scanning and enlarging applications. Anti-Newton glass - also known as AN glass - is a specialist glass product whose surface has been very lightly etched or treated to create a fine, controlled roughness that prevents intimate flat-to-flat contact between the glass and the film surface, ensuring that the air gap between them is too irregular and variable in thickness to support the uniform interference conditions required for Newton's ring formation. Fluid mounting, in which the film is immersed in a thin layer of an optically matched liquid between two glass surfaces, eliminates the air gap entirely by replacing it with a medium whose refractive index closely matches that of the glass and film base, preventing the refractive index discontinuity at which the interfering reflections would otherwise occur. Drum scanners, which wrap the film around a cylindrical drum rather than laying it flat against a glass platen, similarly avoid Newton's rings by ensuring that the curved film surface makes only line contact with the drum rather than the intimate flat contact that promotes ring formation.